A. Field of the Invention
The present invention relates to a lockup clutch of a torque converter, and in particular to a lockup clutch having circumferentially extending coil springs and spring coupling members.
B. Description of the Background Art
A torque converter is a device for transmitting torque from an engine to a transmission via working fluid or oil. A torque converter usually includes an impeller coupled to an input front cover, a turbine opposed to the impeller, and a stator disposed between inner peripheral portions of the impeller and turbine. Working oil flowing from the impeller rotates the turbine, and then is directed by the stator toward the impeller.
Some of the torque converters employ lockup clutches for mechanically transmitting a torque from the input front cover to the turbine or turbine hub. The lockup clutch includes, for example, a piston member which can be coupled to a friction surface of the front cover, a retaining member fixed to the piston member, a plurality of coil springs each carried at its opposite ends by the retaining member, and a circular driven member extending from the turbine hub and engaged with the opposite ends of the coil springs. The coil springs may be replaced with elongated arc-shaped circumferentially extending springs. The arc-shaped springs provide characteristics such as low rigidity and allow for a large maximum torsional displacement angle when the lock-up clutch is engaged, and thus can effectively absorb fine torsional vibrations.
In the conventional lockup clutch, the arc-shaped springs are compressed when a shock or a torsional vibration occurs during a lockup operation, i.e., engagement of the lockup clutch. In this state, the arc-shaped springs are moved radially outward by a centrifugal force, and further the middle portions of them are shifted radially outward relatively to their ends. As a result, the arc-shaped springs slide on an outer peripheral support member. Thereby, a friction occurs between the arc-shaped spring and the outer peripheral support member, so that fine torsional vibrations cannot be sufficiently absorbed due to this friction. Thus, some of the torsional vibrations are transmitted to transmission without being dampened.